Plan Then Build

What will your extension cost — and do you need planning permission?

Free tools for England: get a realistic cost range, then check permitted development in about a minute. Guidance, not legal advice — always confirmed with a Lawful Development Certificate.

Permitted Development & Cost Checker

England only. Takes about a minute.

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By project type

Permitted development, by project type

Each project type sits under a different GPDO 2015 class, with its own depth, height and coverage limits.

ARear extensionASide extensionATwo-storey extensionBLoft conversionBDormer loft conversionEGarden room / outbuildingDPorchEGarage conversion

By region

Extension cost by region

The checker above gives a live estimate for your project. These are the region cost guides behind it.

LondonSouth EastEast of EnglandSouth WestWest MidlandsEast MidlandsNorth WestYorkshire and the HumberNorth East

By council

Council planning data

Real fees, decision times and Article 4 areas for all 296 English councils.

Guidance

Rules explained

What is permitted development?Article 4 directions explainedExtending a house in a conservation areaLawful Development Certificate: cost and processPrior Approval for larger single-storey extensionsThe 50% curtilage coverage ruleLoft conversion and dormer permitted development rulesGarden rooms and outbuildings: permitted development rulesPorch permitted development rulesTwo-storey extension permitted development rules

FAQs

Common questions

Is this legal advice?
No. These tools give guidance based on the GPDO 2015 permitted development rules for England. Always confirm your position with your local planning authority and apply for a Lawful Development Certificate before building.
Does this cover Scotland or Wales?
Not yet. England only for now, since permitted development rules differ by nation.
How accurate is the cost estimate?
It's a ballpark range built from published 2026 UK cost guides and regional multipliers, not a quote. Get itemised quotes from local builders before committing to a budget.
What is a Lawful Development Certificate?
A Lawful Development Certificate (LDC) is a local authority's legal confirmation that a project doesn't need planning permission. It protects you if a neighbour complains or when you sell the house.